This is a Rewind icon. We've made it in iOS style, first introduced in iOS version 7 and supported in all later releases up until now (at least iOS 11). This style is based on thin two-pixel lines and is optimized for 50x50 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 12,600 icons, all in the same style and quality.
The icon is a rewind or back symbol. It consists of two triangles pointing to the left or backwards. Both triangles touch each other, with the second triangle having its apex obscured or abutted into the first triangle.